The spin-lattice relaxation time, $T_{ 1}$, of conduction electrons is measured as a function of temperature and magnetic field in MgB$_2$. The method is based on the detection of the $z$ component of the conduction electron magnetization under electron spin resonance conditions with amplitude modulated microwave excitation. Measurement of $T_{ 1}$ below $T_c$ at 0.32 T allows to disentangle contributions from the two Fermi surfaces of MgB$_{ 2}$ as this field restores normal state on the Fermi surface part with $\pi$ symmetry only.
Categories: cond-mat.supr-con cond-mat.str-el